CVE-2016-8740 in macOS
Summary
by MITRE
The mod_http2 module in the Apache HTTP Server 2.4.17 through 2.4.23, when the Protocols configuration includes h2 or h2c, does not restrict request-header length, which allows remote attackers to cause a denial of service (memory consumption) via crafted CONTINUATION frames in an HTTP/2 request.
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Analysis
by VulDB Data Team • 09/17/2024
The vulnerability identified as CVE-2016-8740 affects the mod_http2 module within Apache HTTP Server versions 2.4.17 through 2.4.23, representing a critical denial of service weakness that specifically targets the HTTP/2 protocol implementation. This flaw exists when the server is configured to accept HTTP/2 connections through the Protocols directive including h2 or h2c protocols, creating an exploitable condition that can be leveraged by remote attackers to consume excessive system resources.
The technical root cause of this vulnerability lies in the improper handling of request-header length restrictions within the HTTP/2 protocol processing pipeline. When HTTP/2 requests are received with CONTINUATION frames containing crafted header data, the mod_http2 module fails to enforce adequate limits on header field sizes. This absence of proper input validation allows attackers to construct malicious HTTP/2 streams with oversized header frames that can cause the server to allocate excessive memory resources without proper bounds checking. The vulnerability specifically exploits the HTTP/2 protocol's frame structure where CONTINUATION frames are used to break header blocks across multiple frames, creating an opportunity for memory exhaustion attacks.
The operational impact of this vulnerability is significant as it enables remote attackers to perform denial of service attacks against Apache HTTP Server instances running vulnerable versions. Attackers can consume large amounts of system memory by sending specially crafted HTTP/2 requests with oversized header data, potentially leading to system instability, service unavailability, and resource exhaustion. This type of attack can be particularly devastating in high-traffic environments where the server's memory resources are already constrained, as the memory consumption can escalate rapidly and cause the server to become unresponsive or crash entirely.
This vulnerability maps directly to CWE-400, which describes unrestricted resource consumption, and aligns with ATT&CK technique T1499.004 for network denial of service attacks. The attack surface is particularly concerning as it affects the core HTTP processing functionality of Apache servers, making it a prime target for automated exploitation. Organizations running vulnerable Apache HTTP Server versions are at risk of experiencing service disruption that can impact availability for legitimate users and potentially provide attackers with opportunities for further exploitation.
The recommended mitigation strategies include immediate upgrade to Apache HTTP Server 2.4.24 or later versions where this vulnerability has been addressed through proper header length validation and memory management controls. Additionally, administrators should implement proper monitoring and alerting mechanisms to detect unusual memory consumption patterns that may indicate exploitation attempts. Network-level controls such as rate limiting and connection tracking can provide additional protection, while disabling HTTP/2 protocol support entirely serves as a temporary workaround for organizations unable to immediately upgrade their systems. The fix implemented in patched versions includes enhanced validation of header frame sizes and proper memory allocation limits that prevent the excessive consumption of system resources during HTTP/2 request processing.